1. Technical Field of the Invention
The present invention is an apparatus and method for horizontal subsea connection of flexible or rigid flowlines and umbilicals or bundles (hereinafter referred to as conduits) to subsea structures.
2. Relevant Art
One of the most complex tasks required to be undertaken by means of diverless intervention is that of connecting conduits to subsea structures. Typically connection is achieved by fitting a termination head of a conduit to a connection hub of a subsea structure and then performing the connection with a subsea connection clamp.
Where the conduit is deployed to the seabed with an end termination head at some time prior to connection, the technique is called On-seabed connection. In such cases, the conduit is pulled to the subsea structure after deployment on the seabed and the termination head connected to the subsea structure at a later date. This type of connection is used predominantly in the North Sea or other offshore environments where the weather conditions are unpredictable. These types of connections are usually compact and, as the connections are horizontal, the size of the seabed structure is usually small.
Deployment of horizontal connections onto the seabed, rather than onto the structure, involves some risk associated with the type of seabed onto which the conduit is to be placed. Deployment on soft seabed will result in sinkage of the conduit termination head, an increase in pull-in loads and reduced visibility. The operation of pulling the termination across the seabed and into the connection position is typically performed by a mobile manipulating device in the form of a toolskid mounted onto an ROV. The ROV provides the electrical or hydraulic power, and buoyancy to allow it to support the toolskid and the termination head of the conduit. Thrusters are provided on the ROV to manoeuvre it and the toolskid around. In general the toolskid functions to pull the termination head of the conduit to the subsea structure and to connect the termination head to the subsea structure and is typically actuated by the ROV through a wetmate connection.
The toolskid is designed to accommodate high pull-in and alignment loads. Consequently toolskids are required to be large enough to cope with these mechanical loads as well as hydrostatic pressure.
On-seabed connection becomes very difficult in deep water. Whilst it is possible to use known, standard ROVs and toolskids up to a depth of 600 m, new designs of ROV and toolskid are required because the toolskids must be larger and more powerful, because the hydrostatic pressure increases with depth. Furthermore, as depth increases, transmission becomes slower and more difficult, and the consequent delay means that the ROV is more difficult to control and higher pull-in and alignment loads are experienced.
Conduits may be connected to subsea structures by docking an ROV and toolskid onto the conduit termination head then flying the termination head to the subsea structure along guide ropes, using a winch and the ROV thrusters. The ROV and toolskid then dock on the subsea structure and connect the conduit to the structure.
U.S. Pat. No. 5,593,249 describes an ROV having a skid frame mounted on it. The ROV is used to ‘fly’ the conduit from its position on the seabed to the subsea structure. The skid frame houses a pair of winches which are used as well as the ROV thrusters to position the open end of a flowline at a suitable position for connection to the subsea structure. In such applications, a large load is borne by the ROV. This makes the apparatus disclosed in U.S. Pat. No. 5,593,249 unsuitable for use in deep water, because of the large hydrostatic pressures and mechanical loads that would be borne by the ROV.
It is also known to use the ROV and the toolskid docked directly on the subsea structure to perform the pull-in operation, using pull-in ropes previously attached to the termination head by the conduit. But during the pull-in operation and the aligning operation, all the loads are borne directly by the toolskid and the ROV which have to be designed in order to resist to these loads.
Another known conduit connection system (U.S. Pat. No. 5,501,549) consists of a housing with a pivotable frame which is used to manipulate the end of the conduit between a slanting pulling position and a final horizontal connection point.
In yet another known conduit connection system, (GB2307288), an ROV is used in conjunction with a lift line that extends from the surface to support a skid and flowline. In this case, it is difficult to control the position of the skid and conduit from the surface. Any heave or pitch occurring at the surface will cause erratic movement of the flowline and may result in damage to the flowline and to the subsea structure.